Power split transmission with energy recovery

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/825,336, filed Sep. 12, 2006, and U.S. Provisional Application No. 60/890,536, filed Feb. 19, 2007, the contents of both are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to power transmission systems. More particularly, this invention relates to power split transmission systems adapted for automotive applications, in which an energy recovery capability is provided by the inclusion of one or more energy storage devices.

Power transmission systems typically found in common automotive applications utilize a mechanical transmission system entirely made up of solid components including shafts, gears, and clutches, which alone may be used to transmit power to the drive wheels of a vehicle, as in the case of “manual” transmissions. Mechanical transmission systems are also used in combination with hydraulic transmission systems that use a liquid under pressure to transmit power, as in the case of an “automatic” transmission that uses a torque converter as a hydrodynamic fluid coupling.

Developments in the automotive industry, including passenger, commercial, and off-road vehicles, are shaped by a strong demand to reduce fuel consumption. In addition, there is a trend toward higher top speed capabilities in heavy off-road vehicles, where faster driving speeds on roads are desired. In response, different types of continuously variable transmissions (CVT) have been developed and brought to the market. Among the CVT concepts, hybrid drives are of particular interest to vehicle manufactures. Hybrid drives are based on the utilization of brake energy for vehicle propulsion by storing brake energy in a battery, fly wheel, hydraulic accumulator, or other energy storage means. By recapturing energy otherwise lost as heat during braking, the hybrid drive technology is capable of significantly reducing fuel consumption, particularly in heavy trucks and cars. Comprehensive overviews of hydraulic hybrid drives are provided in Stecki et al., “Advances in Automotive Hydraulic Hybrid Drives,” Proceedings of the Sixth International Conference on Fluid Power Transmission and Control, Hangzhou, China (2005), and Miller, “Comparative Assessment of Hybrid Vehicle Power Split Transmissions,” 4th VI Winter Workshop Series (2005). While CVT's provide seamless shifting in vehicle operation, allowing the engine to operate at a nominal speed range resulting in lower fuel consumption and emissions, typical CVT's suffer from either low shaft-to-shaft efficiency or low torque handling capabilities.

Power split transmissions (PST), also known as power split drive (PSD) transmissions, are a particular type of CVT that has found use in applications such as agricultural tractors, for example, the Fendt Vario line of tractors brought to the market by Fendt (AGCO Corporation) in 1996. See, for example, Dziuba et al., “Entwicklung eines neuen stufenlosen Schleppergetriebes mit hydrostatisch mechanischer Leistungsverzweigung,” VDI-Berichte Nr. 1393, VDI-Verlag, Düsseldorf, Germany (1998), p. 541-549 (in German). Although the principle of power split drives has been known for more than four decades, this technology is still in a developmental stage.

As known, PSD transmissions traditionally use a planetary (epicyclic) gear train (PGT) in combination with a continuously variable transmission that achieves continuous variable speed control along with high efficiency levels that are derived from the mechanical gears of the PGT. There are three basic implementations of power split drives: the combination of a PGT with a continuous variable mechanical gear; the combination of a PGT with a hydrodynamic transmission; and the combination of a PGT with a hydrostatic transmission. The last of these allows further fuel savings if a drive line control concept is implemented that takes engine characteristics into account. The engine speed can be adjusted to a point where the total power loss of the transmission is minimized, as reported in Ossyra et al., “Drive Line Control for Off-Road Vehicles Helps to Save Fuel,” SAE International Commercial Vehicle Engineering Congress, Chicago, Ill., USA, SAE Technical Paper 2004-01-2673 (2004). Through the improvement of the efficiency of positive displacement machines, the use of hydrostatic transmissions in PSD's has become very attractive for many different applications.

PSD transmissions have three different operating modes that are known in the automotive transmission industry under the following designations: power additive, full mechanical, and power recirculation. The power flows of these three modes are schematically represented in <figref idrefs="DRAWINGS">FIG. 1</figref>. In the power additive mode, the power, P_in, from a combustion engine (or other suitable power source) is split and transferred into two paths: a mechanical path (containing a planetary gear train), P_mech, and a hydraulic path (such as a hydrostatic transmission), P_hyd. The power is then combined and transferred as P_outto the wheels to propel a vehicle. In full mechanical mode, power is entirely transferred from the engine to the wheels via the mechanical path (P_mech) and not through the hydraulic path (P_hyd). Generally full mechanical mode is at a single speed or has a small speed range. In the power recirculation mode, some of the power transferred via the mechanical path (P_mech) is recirculated back through the hydraulic path (P_hyd). The recirculated hydraulic power (P_hyd) is combined with the engine power (P_in) from the engine and again transferred via the mechanical path (P_mech), thus being recirculated. In general, full mechanical mode is considered to be the most efficient transmission power mode for a PSD transmission, and the power recirculation mode is considered to be the least efficient transmission power mode because large amounts of power can be recirculated through the hydraulic path.

<figref idrefs="DRAWINGS">FIG. 2</figref> identifies PSD transmissions categorized by families based on structural similarities—first, whether the hydraulic path (P_hyd) is coupled to the input (input coupled) or to the output (output coupled) of the mechanical transmission system, and then further categorized by the characteristics of the planetary gear train (basic, multistage, or compound). A comparison of achievable efficiencies and operating characteristics of these structural approaches has been presented in Carl et al., “Comparison of Operational Characteristics in Power Split Continuously Variable Transmissions,” SAE Commercial Vehicle Engineering Congress and Exhibition, Chicago, USA, SAE Technical Paper 2006-01-3468 (2006).

The basic output and input-coupled types are represented in <figref idrefs="DRAWINGS">FIGS. 3 and 8</figref>, respectively. <figref idrefs="DRAWINGS">FIG. 3</figref> shows a basic output-coupled PSD transmission <b>10</b> as utilizing a hydrostatic transmission <b>12</b> as the hydraulic path and continuously variable part of the transmission <b>10</b>. The hydrostatic transmission <b>12</b> is mechanically coupled (via a gear set) to the output of a simple planetary gear train <b>14</b> (i.e., not multistage or compound), which serves as the mechanical transmission system (path) of the transmission <b>10</b>. The planetary gear train <b>14</b> is mechanically coupled (via a shaft) to a combustion engine <b>11</b> as the power source of the vehicle in which the transmission <b>10</b> is installed. The outputs of the hydrostatic transmission <b>12</b> and planetary gear train <b>14</b> are both mechanically coupled (via a gear set and a shaft, respectively) to the drive axle and wheels <b>19</b> of the vehicle. The hydrostatic transmission <b>12</b> comprises two positive displacement units <b>16</b> and <b>18</b>, labeled “Unit <b>1</b>” and “Unit <b>2</b>” in <figref idrefs="DRAWINGS">FIG. 3</figref>. As understood in the art, the positive displacement units <b>16</b> and <b>18</b> operate by trapping and then displacing a fixed volume of hydraulic fluid. As such, the speed of the vehicle can be controlled by controlling the displacements of the units <b>16</b> and <b>18</b> using the vehicle velocity as a feedback signal. The output-coupled transmission <b>10</b> of <figref idrefs="DRAWINGS">FIG. 3</figref> is limited to two operational modes: power additive and power recirculation. The output-coupled transmission <b>10</b> operates in the power additive mode at low speeds and the power recirculation mode at high speeds. With constant engine speed, increasing the vehicle forward velocity from standstill is achieved by increasing the displacement of the unit <b>16</b> from zero to maximum, then decreasing the displacement of the unit <b>18</b> from maximum to zero. Reverse is achieved by running the unit <b>16</b> over center. The differential pressure in the hydrostatic transmission <b>12</b> is determined by the load torque. During braking, the high pressure and low pressure lines switch as the units <b>16</b> and <b>18</b> change pumping and motoring modes.

In <figref idrefs="DRAWINGS">FIG. 8</figref>, a basic input-coupled PSD transmission <b>50</b> is shown as utilizing a hydrostatic transmission <b>52</b> as the hydraulic path and variable part of the transmission <b>50</b>, and a planetary gear train <b>54</b> that serves as the mechanical path of the transmission <b>50</b>. In contrast to the output-coupled PSD transmission <b>10</b> of <figref idrefs="DRAWINGS">FIG. 3</figref>, the hydrostatic transmission <b>52</b> and the planetary gear train <b>54</b> are both mechanically coupled (via a gear set and a shaft, respectively) to a combustion engine <b>51</b>, the hydrostatic transmission <b>52</b> is mechanically coupled (via a gear set) to the input of the planetary gear train <b>54</b>, and the output of only the planetary gear train <b>54</b> is mechanically coupled (via a shaft) to the drive axle and wheels <b>59</b> of the vehicle. Similar to the transmission <b>10</b> of <figref idrefs="DRAWINGS">FIG. 3</figref>, the speed of the vehicle is controlled by controlling the displacements of two positive displacement units <b>56</b> and <b>58</b> of the hydrostatic transmission <b>52</b>, labeled “Unit I” and “Unit II” in <figref idrefs="DRAWINGS">FIG. 8</figref>, using the vehicle speed as feedback.

As with the output-coupled transmission <b>10</b> of <figref idrefs="DRAWINGS">FIG. 3</figref>, the input-coupled transmission <b>50</b> is limited to two operational modes: power additive and power recirculation. The transmission <b>50</b> operates in power recirculation mode at low speeds, and at higher speeds operates in power additive mode. This cycle can be repeated several times by adding clutches and more advanced planetary gear trains (e.g., multistage and compound). With constant engine speed, increasing the vehicle forward velocity from standstill is achieved by increasing the displacement of the unit <b>56</b> from negative maximum through zero to positive maximum, then decreasing the displacement of the unit <b>58</b>. Reverse is achieved by holding the displacement of the unit <b>56</b> at maximum and decreasing the displacement of the unit <b>58</b>. The differential pressure within the hydrostatic transmission system <b>52</b> is simply a reactionary item, a function of the load torque on the wheels <b>59</b>. Deceleration is only possible with standard friction-type brakes connected to the wheel axle.

In view of the above, both the output-coupled and input-coupled PSD transmissions have certain limitations and inefficiencies, such that additional developments and improvements would be desirable to further expand the technical and commercial viability of PSD transmissions.

BRIEF SUMMARY OF THE INVENTION

The present invention provides power split drive (PSD) transmissions that are suitable for use in automotive applications, and exhibit improved operating efficiencies as a result of having an energy recovery capability.

A PSD transmission of this invention includes a mechanical transmission system for mechanically transmitting mechanical power between a rotatable input shaft (for example, a shaft connected to an engine) and a rotatable output shaft (for example, a shaft connected to a wheel axle), and a hydraulic transmission system containing a fluid for hydraulically transmitting hydraulic power between the input shaft and the output shaft, and at least a third shaft operatively interconnected to one of the mechanical and hydraulic transmission systems. The hydraulic transmission system is operatively coupled by at least a first planetary gear train to the mechanical transmission system. According to the invention, the PSD further comprises means operatively associated with at least one of the mechanical and hydraulic transmission systems for storing and releasing energy within the PSD transmission, the energy storing and releasing means comprising a flywheel or an accumulator or a combination thereof.

According to one aspect of the invention, the energy storing means comprises one or more flywheels and/or accumulators. In the case of the former, the flywheel may be coupled to the input shaft and/or the third shaft. In the case of the latter, at least one accumulator is preferably fluidically coupled to a positive displacement device operable to store energy from the power split transmission by operating as a pump to store a portion of the fluid at an elevated pressure in the accumulator and operable to release energy to the power split transmission by operating as a motor driven by the fluid released from the accumulator. PSD transmissions of this invention are preferably capable of combining the variability of a CVT and the efficiency of mechanical transmission systems, along with an energy storage capability that provides potential benefits for both on road and off road vehicles.

According to additional aspects of the invention, the PSD transmission can by categorized based on whether the hydraulic transmission system (defining the hydraulic path of the PSD transmission) is coupled to the input (input coupled) or to the output (output coupled) of the mechanical transmission system, and the characteristics of the planetary gear train (basic, dual-stage, or compound). Accordingly, the PSD transmission may be a basic output-coupled embodiment in which case the hydraulic transmission system is mechanically coupled through the first planetary gear train to the input shaft and mechanically coupled to the output shaft, or a compound output-coupled embodiment in which case the hydraulic transmission system is mechanically coupled through the first planetary gear train to the input shaft and mechanically coupled through a second planetary gear train to the output shaft, or a basic input-coupled embodiment in which case the hydraulic transmission system is mechanically coupled to the input shaft and mechanically coupled through the first planetary gear train to the output shaft, or a multistage input-coupled embodiment in which case the hydraulic transmission system is mechanically coupled to the input shaft and the first planetary gear train is a multistage planetary gear train that mechanically couples the hydraulic transmission system to the output shaft.

Other objects and advantages of this invention will be better appreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idrefs="DRAWINGS">FIGS. 1 and 2</figref> are charts showing the basic power modes and transmission families of PSD transmissions.

<figref idrefs="DRAWINGS">FIG. 3</figref> schematically represents a basic output-coupled PSD transmission known in the prior art.

<figref idrefs="DRAWINGS">FIG. 4</figref> schematically represents a basic output-coupled PSD transmission equipped with a flywheel and accumulator that provide an energy recovery capability in accordance with a first embodiment of this invention.

<figref idrefs="DRAWINGS">FIG. 5</figref> schematically represents a compound output-coupled PSD transmission equipped with a flywheel and accumulator that provide an energy recovery capability in accordance with a second embodiment of this invention.

<figref idrefs="DRAWINGS">FIG. 6</figref> schematically represents a basic output-coupled PSD transmission equipped with only an accumulator to provide an energy recovery capability in accordance with a third embodiment of this invention.

<figref idrefs="DRAWINGS">FIG. 7</figref> schematically represents a compound output-coupled PSD transmission equipped with only an accumulator to provide an energy recovery capability in accordance with a fourth embodiment of this invention.

<figref idrefs="DRAWINGS">FIG. 8</figref> schematically represents a basic input-coupled PSD transmission known in the prior art.

<figref idrefs="DRAWINGS">FIG. 9</figref> schematically represents a basic input-coupled PSD transmission equipped with a flywheel and accumulator that provide an energy recovery capability in accordance with a fifth embodiment of this invention.

<figref idrefs="DRAWINGS">FIG. 10</figref> schematically represents a multistage input-coupled PSD transmission equipped with a flywheel and accumulator that provide an energy recovery capability in accordance with a sixth embodiment of this invention.

<figref idrefs="DRAWINGS">FIG. 11</figref> schematically represents a basic input-coupled PSD transmission equipped with only an accumulator to provide an energy recovery capability in accordance with a seventh embodiment of this invention.

<figref idrefs="DRAWINGS">FIG. 12</figref> schematically represents a multistage input-coupled PSD transmission equipped with only an accumulator to provide an energy recovery capability in accordance with an eighth embodiment of this invention.

<figref idrefs="DRAWINGS">FIGS. 13A through 13D</figref> identify four power flow modes of output-coupled PSD transmissions of this invention.

<figref idrefs="DRAWINGS">FIGS. 14A through 14G</figref> identify eight power flow modes of input-coupled PSD transmissions of this invention.

<figref idrefs="DRAWINGS">FIG. 15</figref> schematically represents Standard Control and Secondary Control schemes for use with the input-coupled PSD transmissions of this invention.

DETAILED DESCRIPTION OF THE INVENTION

<figref idrefs="DRAWINGS">FIGS. 4 through 7</figref> and <b>9</b> through <b>12</b> schematically represent PSD transmissions (PST's) comprising mechanical and hydrostatic transmission systems, in which the hydrostatic transmission systems (the hydraulic path of the PSD transmission) are coupled to either the output (output-coupled) or input (input-coupled) of its mechanical transmission system. In each case, the PSD transmission is capable of capturing and releasing energy, preferably over the entire speed range of the PSD transmission, through the use of a storage recovery and release system that comprises one or more accumulators integrated into the hydrostatic transmission system and/or one or more flywheels integrated into the mechanical transmission system. Because the invention takes the form of multiple embodiments that employ functionally similar components, consistent reference numbers are used where noted to identify functionally similar components.

<figref idrefs="DRAWINGS">FIGS. 4 through 7</figref> represent output-coupled PSD transmissions with energy recovery capabilities according to four embodiments of the invention. <figref idrefs="DRAWINGS">FIG. 4</figref> represents a basic output-coupled PSD transmission <b>110</b> of this invention, with energy storage capability provided in the form of two accumulators <b>120</b> and <b>122</b> and an optional flywheel <b>124</b>. The flywheel <b>124</b> is mechanically coupled to a combustion engine <b>111</b> (or other suitable power source) through a freewheel clutch <b>126</b> and is mounted on an engine-driven shaft <b>128</b> to a planetary (epicyclic) gear train <b>114</b> that forms part of the mechanical transmission system of the PSD transmission <b>110</b>. As with conventional planetary gear trains, the planetary gear train <b>114</b> is represented as comprising a sun gear <b>130</b>, a ring gear <b>132</b> circumscribing the sun gear <b>130</b>, and planet gears <b>134</b> carried on a planet gear carrier <b>136</b> and simultaneously in mesh with the sun gear <b>130</b> and ring gear <b>132</b>. In <figref idrefs="DRAWINGS">FIG. 4</figref>, the planet gear carrier <b>136</b> is shown coupled to the engine-driven shaft <b>128</b> and the sun gear <b>130</b> is mounted on an output shaft <b>138</b> of the mechanical transmission system coupled to the drive axle and wheels <b>119</b> of the vehicle.

The accumulators <b>120</b> and <b>122</b> are high pressure (HP) and low pressure (LP) accumulators, respectively, integrated into a hydrostatic transmission system <b>112</b> of the PSD transmission <b>110</b>. The hydrostatic transmission system <b>112</b> further includes first and second positive displacement units <b>116</b> and <b>118</b> (Units <b>1</b> and <b>2</b>), respectively, each coupled to fluid lines A and B. The first and second units <b>116</b> and <b>118</b> are coupled to the ring gear <b>132</b> and output shaft <b>138</b>, respectively, with shafts <b>140</b> and <b>142</b> and suitable gearing (having gear ratios of i₁and i₂, respectively). The high pressure and low pressure accumulators <b>120</b> and <b>122</b> are directly connected to the fluid lines A and line B, respectively. Other than pressure relief valves (not shown), additional valves could be used but are not required for the embodiment of <figref idrefs="DRAWINGS">FIG. 4</figref> as shown.

<figref idrefs="DRAWINGS">FIG. 5</figref> shows a compound PSD transmission <b>210</b> that is essentially the same as the basic PSD transmission <b>110</b> of <figref idrefs="DRAWINGS">FIG. 4</figref> (hence, the usage of the same reference numbers for its components), but with the further addition of a second planetary gear train <b>115</b> through which the hydrostatic transmission system <b>112</b> is coupled to the output shaft <b>138</b>, instead of the shaft <b>142</b> and gearing of <figref idrefs="DRAWINGS">FIG. 4</figref>. <figref idrefs="DRAWINGS">FIGS. 6 and 7</figref> further show basic and compound variations <b>310</b> and <b>410</b>, respectively, of the PSD transmission <b>110</b> of <figref idrefs="DRAWINGS">FIG. 4</figref> (hence, the usage of the same reference numbers for their components), but with energy storage means <b>120</b>/<b>122</b>/<b>124</b> coupled to the engine-driven shaft <b>128</b> by a shaft <b>144</b> and suitable gearing. While the energy storage means <b>120</b>/<b>122</b>/<b>124</b> is shown in <figref idrefs="DRAWINGS">FIGS. 6 and 7</figref> as a single accumulator, one or more flywheels (with suitable clutches) could be used, as could combinations of accumulator(s) and flywheel(s).

With the inclusion of the flywheel <b>124</b> coupled to the engine-driven shaft <b>128</b> and/or the accumulators <b>120</b> and <b>122</b> within the hydrostatic transmission system <b>112</b>, brake energy can be stored and used for vehicle propulsion. There are three main types of power flow modes associated with the output-coupled PSD transmissions of <figref idrefs="DRAWINGS">FIGS. 4 through 7</figref>: propulsion via the engine <b>111</b> or flywheel <b>124</b>; propulsion via the accumulator(s) <b>120</b>/<b>122</b>; and energy storage using the flywheel <b>124</b> and/or accumulator(s) <b>120</b>/<b>122</b>. Energy recovery can occur in both the additive and recirculation operating modes of the PSD transmissions <b>110</b>, <b>210</b>, <b>310</b>, and <b>410</b>.

In the power additive mode, propulsion of the vehicle can be accomplished through the use of power from the engine <b>111</b> or the flywheel <b>124</b>. The power from the engine/flywheel <b>111</b>/<b>124</b> is delivered by the shaft <b>128</b> to the planet gear carrier <b>136</b> of the planetary gear train <b>114</b>. Within the gear train <b>114</b>, the power is split between a shaft <b>146</b> (coupled to the ring gear <b>132</b>) and the output shaft <b>138</b> (coupled to the sun gear <b>130</b>). The power in the output shaft <b>138</b> is transferred to the wheels <b>119</b> through a mechanical path (e.g., the shaft <b>138</b> and wheel axle), while the power in the shaft <b>146</b> is transferred to the hydrostatic transmission system <b>112</b> (via the associated gear set). Within the hydrostatic transmission system <b>112</b>, the displacement units <b>116</b> and <b>118</b> operate as a pump and motor, respectively, when the transmission <b>110</b> is in the power additive mode.

In the power additive mode, propulsion of the vehicle can also be accomplished through the use of stored energy from the accumulators <b>120</b> and <b>122</b>. The stored energy is in the form of pressurized fluid within the HP accumulator <b>120</b> and is transferred to the unit <b>118</b>, which acts as a motor to drive the shaft <b>142</b>. The capability of driving the displacement unit <b>118</b> with pressurized fluid to propel the vehicle requires a change in the control of the displacements of the units <b>116</b> and <b>118</b> beyond that necessary for the prior art output-coupled PSD transmission <b>10</b> of <figref idrefs="DRAWINGS">FIG. 3</figref>.

Energy storage can also occur in the power additive mode using the flywheel <b>124</b> and/or accumulator(s) <b>120</b>/<b>122</b>. When braking is desired, power is transferred to the hydrostatic transmission system <b>112</b> through the gearing connecting the shaft <b>142</b> to the output shaft <b>138</b>, and transferred through the output shaft <b>138</b> to the planetary gear train <b>114</b>. When storing energy with the accumulators <b>120</b> and <b>122</b>, the displacement unit <b>118</b> operates as a pump and the displacement unit <b>116</b> operates at zero displacement. When storing energy with the flywheel <b>124</b>, the unit <b>118</b> operates as a pump and the unit <b>116</b> operates as a motor, transferring power to the flywheel <b>124</b> via the shaft <b>146</b> and planetary gear train <b>114</b>. The freewheel clutch <b>126</b> allows the flywheel <b>124</b> to accelerate and rotate at speeds faster than the engine <b>111</b>, thus increasing the energy storage capability of the flywheel <b>124</b>.

In the power recirculation mode, propulsion of the vehicle can be accomplished through the use of power from the engine <b>111</b> or the flywheel <b>124</b>. The power from the engine/flywheel <b>111</b>/<b>124</b> is summed in the planetary gear train <b>114</b> through the shafts <b>128</b> and <b>138</b>. The power delivered by the shaft <b>138</b> is split at gearing that couples the shaft <b>138</b> to the shaft <b>142</b> of the displacement unit <b>118</b>, with part of the power being transferred to the wheels <b>119</b> to propel the vehicle and the remainder recirculated through the hydrostatic transmission system <b>112</b> to allow for further increase of vehicle speed. Within the hydrostatic transmission system <b>112</b>, the displacement unit <b>118</b> operates as a motor driven by the fluid output of the displacement unit <b>116</b>.

In the power recirculation mode, propulsion of the vehicle can also be accomplished through the use of stored energy from the HP accumulator <b>120</b>, whose pressurized fluid is transferred to the displacement unit <b>118</b> acting as a motor.

Finally, the power recirculation mode also allows for energy storage using the flywheel <b>124</b> and/or accumulators <b>120</b>/<b>122</b>. When braking is desired, power is transferred to the hydrostatic transmission system <b>112</b> through the gearing connecting the shafts <b>138</b> and <b>142</b>, and transferred through the shaft <b>138</b> to the planetary gear train <b>114</b>. When storing energy with the HP accumulator <b>120</b>, the displacement unit <b>118</b> operates as a pump and the displacement unit <b>116</b> operates at either zero displacement or as a pump. When storing energy with the flywheel <b>124</b>, the displacement unit <b>116</b> operates as a pump and the mode of the displacement unit <b>118</b> determines if power is recirculated to the shaft <b>138</b> or stored in the accumulator <b>120</b>. Again, the freewheel clutch <b>126</b> allows the flywheel <b>124</b> to accelerate and rotate at speeds faster than the engine <b>111</b>.

In view of the above, in contrast to the conventional output-coupled transmission <b>10</b> shown in <figref idrefs="DRAWINGS">FIG. 3</figref>, energy recovery with the PSD transmissions <b>110</b>, <b>210</b>, <b>310</b>, and <b>410</b> of <figref idrefs="DRAWINGS">FIGS. 4 through 7</figref> involves the use of different control of the displacement units <b>116</b> and <b>118</b>. The displacement unit <b>116</b> is used to control the pressure in line A based on a lowest desired minimum pressure corresponding to the minimum fluid volume in the high pressure accumulator <b>120</b>, and the displacement unit <b>118</b> is used to control the vehicle speed based on the desired vehicle speed. Therefore, as in the conventional output-coupled transmission of <figref idrefs="DRAWINGS">FIG. 3</figref>, a feedback signal from the vehicle speed is necessary for controlling the displacement unit <b>118</b>. Additionally, a pressure signal from line A is employed to control the displacement unit <b>116</b>.

As discussed above and represented in further detail in <figref idrefs="DRAWINGS">FIGS. 13A through 13D</figref>, the displacement unit <b>116</b> either operates as a pump or is freewheeling, while the displacement unit <b>118</b> either operates as a motor or pump. As such, the displacement unit <b>118</b> is an over-center unit that allows its operation as a motor or pump, while the displacement unit <b>116</b> may but does not require an over-center operation capability. The implementation of the described control concept, referred to as the secondary control principle, allows the implementation of the high and low pressure accumulators <b>120</b> and <b>122</b> for use in energy recovery without the need for switching valves in fluid lines A and B. However, the use of switching valves could be used and is therefore also within the scope of the invention.

The basic output-coupled PSD transmission <b>110</b> of <figref idrefs="DRAWINGS">FIG. 4</figref> is designed in such a way that in forward speeds the transmission <b>110</b> runs solely in power additive mode, without ever entering full mechanical mode or power recirculation mode. As full mechanical mode is approached the speed of the displacement unit <b>116</b> decreases to zero. In the secondary controlled system of the output-coupled PSD transmission <b>110</b>, this will result in the emptying of the accumulator <b>120</b> as flow exits to the displacement unit <b>118</b> but no flow is produced by the unit <b>116</b>. In reverse the transmission <b>110</b> operates in power recirculation mode, with the unit <b>118</b> operating over center as a motor. Due to the direct coupling of the unit <b>118</b> to the shaft <b>138</b> and wheels <b>119</b>, energy capture is possible in all speed ranges.

The operational modes are shown in more detail in <figref idrefs="DRAWINGS">FIGS. 13A through 13D</figref> and discussed below in particular reference to the basic output-coupled PSD transmission <b>110</b> of <figref idrefs="DRAWINGS">FIG. 4</figref>, though it should be understood that the operating principles also apply to the transmissions <b>210</b>, <b>310</b>, and <b>410</b> of <figref idrefs="DRAWINGS">FIGS. 5 through 7</figref>. During propulsion (Power Flow Modes <b>0</b>, I, and II in <figref idrefs="DRAWINGS">FIGS. 13A</figref>, <b>13</b>B and <b>13</b>C, respectively) and braking (Power Flow Mode III in <figref idrefs="DRAWINGS">FIG. 13D</figref>), the fluid line A is always the high pressure line and fluid line B is always the low pressure line. At zero vehicle speed, v_veh, the displacement of the unit <b>118</b> is zero. As the desired vehicle speed (v_veh,des) is increased, the displacement of unit <b>118</b> increases, delivering a torque to the wheels <b>119</b>. In propulsion, the vehicle may be powered by either the engine <b>111</b> or the high pressure accumulator <b>122</b>.

In propulsion, if the pressure in line A is at the minimum operating pressure (p_1HP), the transmission <b>110</b> will be running as a PSD transmission powered by the engine <b>111</b> (Power Flow Mode I in <figref idrefs="DRAWINGS">FIG. 13B</figref>). Unit <b>116</b> is in pumping mode and unit <b>118</b> is in motoring mode. Unit <b>116</b> holds the pressure in the fluid line A at the minimum operating pressure, p_1HP. As the pressure in line A drops below p_1HP, the displacement of unit <b>116</b> increases, filling the accumulator <b>120</b> and increasing the pressure in line A. As v_veh,desincreases, unit <b>118</b> increases in displacement, producing a torque on the wheels <b>119</b> of the vehicle. Power is transferred from the engine-driven shaft <b>128</b> both mechanically (PA) and hydraulically (PC). If, in propulsion, the pressure in line A is higher than p_1HP, the transmission <b>110</b> is powered solely by the high pressure accumulator <b>120</b> (Power Flow Mode II in <figref idrefs="DRAWINGS">FIG. 13C</figref>). The unit <b>116</b> is freewheeling and the unit <b>118</b> is in motoring mode. The unit <b>116</b> is controlled by the pressure in line A to zero displacement, and the unit <b>118</b> is controlled by v_veh,des. No power is transferred from the engine <b>111</b>.

During braking (Power Flow Mode III in <figref idrefs="DRAWINGS">FIG. 13D</figref>), the unit <b>118</b> operates over center, producing a negative torque at the wheels <b>119</b> and acting in pumping mode. As the pressure in line A increases above p_1HP, unit <b>116</b> is controlled to zero displacement, and flow from unit <b>118</b> is used to charge the high pressure accumulator <b>120</b> to store brake energy. The displacement of unit <b>118</b>, and therefore the amount of brake torque, is controlled by v_veh,des. If the high pressure accumulator <b>120</b> is filled, fluid flow from the unit <b>118</b> is directed to the low pressure accumulator <b>122</b> through a pressure relief valve. Therefore, the size of the accumulators <b>120</b> and <b>122</b> play an important role in brake energy recovery. No power is transferred back to unit <b>116</b> or the shaft <b>128</b>. The use of additional braking may be necessary for both safety and high braking demands.

Reverse is accomplished by controlling the displacement unit <b>118</b> over center. If the pressure in fluid line A is at p_1HP, the transmission <b>110</b> runs in power recirculation mode (Power Flow Mode <b>0</b> in <figref idrefs="DRAWINGS">FIG. 13A</figref>). If the pressure in line A is above p_1HP, the vehicle will be powered by the high pressure accumulator <b>120</b>, as in Power Flow Mode II (<figref idrefs="DRAWINGS">FIG. 13C</figref>).

<figref idrefs="DRAWINGS">FIGS. 9 through 12</figref> represent input-coupled PSD transmissions with energy recovery capabilities according to four additional embodiments of the invention. <figref idrefs="DRAWINGS">FIG. 9</figref> represents a basic input-coupled PSD transmission <b>150</b> of this invention, with the energy storage capability provided in the form of two accumulators <b>160</b> and <b>162</b> and an optional flywheel <b>164</b>. The flywheel <b>164</b> is mechanically coupled to a combustion engine <b>151</b> (or other suitable power source) through a freewheel clutch <b>166</b> that allows the engine <b>151</b> to drop to speeds lower than the rotational speed of the shaft <b>168</b> on which the flywheel <b>164</b> is mounted. The shaft <b>168</b> is coupled to a planetary (epicyclic) gear train <b>154</b> that forms part of the mechanical transmission system of the PSD transmission <b>150</b>. As with conventional planetary gear trains, the planetary gear train <b>154</b> is represented as comprising a sun gear <b>170</b>, a ring gear <b>172</b> circumscribing the sun gear <b>170</b>, and planet gears <b>174</b> carried on a planet gear carrier <b>176</b> and simultaneously in mesh with the sun gear <b>170</b> and ring gear <b>172</b>. In contrast to the output-coupled PSD transmissions <b>110</b>, <b>210</b>, <b>310</b>, and <b>410</b> of <figref idrefs="DRAWINGS">FIGS. 4 through 7</figref>, the input-coupled PSD transmission <b>150</b> of <figref idrefs="DRAWINGS">FIG. 9</figref> has its sun gear <b>170</b> coupled to the shaft <b>168</b> and its planet gear carrier <b>176</b> mounted on a shaft <b>178</b> coupled to the drive axle and wheels <b>159</b> of the vehicle.

The accumulators <b>160</b> and <b>162</b> are high pressure (HP) and low pressure (LP) accumulators, respectively, integrated into a hydrostatic transmission system <b>152</b> of the PSD transmission <b>150</b>. The high pressure accumulator <b>160</b> is for energy storage and the low pressure accumulator <b>162</b> is for low pressure fluid storage. The hydrostatic transmission system <b>152</b> further includes first and second positive displacement units <b>156</b> and <b>158</b> (Units <b>1</b> and <b>2</b>), respectively, each coupled to fluid lines A and B. The first and second units <b>156</b> and <b>158</b> are coupled to the ring gear <b>172</b> and shaft <b>168</b>, respectively, with shafts <b>180</b> and <b>182</b> and suitable gearing. The high pressure and low pressure accumulators <b>162</b> and <b>164</b> are connected to the fluid lines A and line B through a valve block <b>163</b> whose components are schematically detailed in <figref idrefs="DRAWINGS">FIG. 15</figref>. Other than pressure relief valves (not shown), additional valves may be used but are not required for the embodiment of <figref idrefs="DRAWINGS">FIG. 9</figref> as shown.

<figref idrefs="DRAWINGS">FIG. 10</figref> shows a multistage input-coupled PSD transmission <b>250</b> that is essentially the same as the basic input-coupled PSD transmission <b>150</b> of <figref idrefs="DRAWINGS">FIG. 9</figref> (hence, the usage of the same reference numbers for its components), except that the planetary gear train <b>154</b> is a dual-stage planetary gear train capable of smaller or larger gear ratios, as known in the art. <figref idrefs="DRAWINGS">FIGS. 11 and 12</figref> further show basic and multistage variations <b>350</b> and <b>450</b>, respectively, of the PSD transmission <b>150</b> of <figref idrefs="DRAWINGS">FIG. 9</figref> (hence, the usage of the same reference numbers for their components), but with energy storage means <b>160</b>/<b>162</b>/<b>164</b> coupled to the engine-driven shaft <b>168</b> by a shaft <b>184</b> and suitable gearing. While the energy storage means <b>160</b>/<b>162</b>/<b>164</b> is represented in <figref idrefs="DRAWINGS">FIGS. 11 and 12</figref> as a single accumulator, one or more flywheels (with clutches) could be used, as could combinations of accumulator(s) and flywheel(s).

With the inclusion of the flywheel <b>164</b> coupled to the engine-driven shaft <b>168</b> and/or the accumulators <b>160</b> and <b>162</b> within the hydrostatic transmission system <b>152</b>, brake energy can be stored and used for vehicle propulsion. Three main types of power flow modes are associated with the input-coupled PSD transmissions of <figref idrefs="DRAWINGS">FIGS. 9 through 12</figref>: propulsion via the engine <b>151</b> or flywheel <b>164</b>; propulsion via the accumulator(s) <b>160</b>/<b>162</b>; and braking using the flywheel <b>164</b> and/or the accumulator(s) <b>160</b>/<b>162</b>. It is also possible to combine some forms of propulsion, for example, propulsion can also occur via a combination of the engine <b>151</b> and accumulator(s) <b>160</b>/<b>162</b>, or a combination of the flywheel <b>164</b> and accumulators <b>160</b>/<b>162</b>. Energy recovery can occur in both the additive and recirculation operating modes of the PSD transmissions <b>150</b>, <b>250</b>, <b>350</b>, and <b>450</b>.

In the power recirculation mode, propulsion of the vehicle can be accomplished through the use of power from the engine <b>151</b> or the flywheel <b>164</b>. The power from the engine/flywheel <b>151</b>/<b>164</b> is carried through the shaft <b>168</b> and split in the planetary gear train <b>154</b> to the shaft <b>178</b> (coupled to the planet gear carrier <b>176</b>) and a shaft <b>186</b> coupled to the hydrostatic transmission system <b>152</b> through the ring gear <b>172</b>. The power at the shaft <b>178</b> is transferred to the wheels <b>159</b> to propel the vehicle, while the remainder is recirculated through the hydrostatic transmission system <b>152</b>. Within the hydrostatic transmission system <b>152</b>, the displacement unit <b>158</b> operates as a pump and the displacement unit <b>156</b> operates as a motor.

The power recirculation mode also allows for energy storage using the flywheel <b>164</b>. When braking is desired, power is transferred from the shaft <b>178</b> to the engine shaft <b>168</b>, which drives both the flywheel <b>164</b> and the displacement unit <b>156</b>. The unit <b>156</b> operates as a pump while the unit <b>158</b> operates as a motor, transferring power to the shaft <b>186</b>. Power from the shafts <b>178</b> and <b>186</b> is summed in the planetary gear train <b>154</b> and recirculated to the shaft <b>168</b>. The freewheel clutch <b>166</b> allows the flywheel <b>164</b> to accelerate and rotate at speeds faster than the engine <b>151</b>.

In the power additive mode, propulsion of the vehicle can be accomplished through the use of power from the engine <b>151</b> or the flywheel <b>164</b>. Power from the engine/flywheel <b>151</b>/<b>164</b> is summed with the power of the hydrostatic transmission system <b>152</b> in the planetary gear train <b>154</b> through the shaft <b>168</b> (coupled to the sun gear <b>130</b>) and shaft <b>186</b> (coupled to the ring gear <b>132</b>). Within the hydrostatic transmission system <b>152</b>, the displacement unit <b>156</b> operates as a pump and the displacement unit <b>158</b> operates as a motor.

Propulsion of the vehicle can also be accomplished in the power additive mode through the use of stored energy from the HP accumulator <b>160</b>. The stored energy is transferred to the units <b>156</b> and <b>158</b>, both of which operate as motors to drive, respectively, the shaft <b>180</b> coupled to the shaft <b>168</b> and the shaft <b>182</b> coupled to the ring gear <b>172</b>. If the flywheel <b>164</b> also has stored energy, power from the flywheel <b>164</b> and HP accumulator <b>160</b> can be used to propel the vehicle. Such a capability entails a change to the control of the displacements of the units <b>156</b> and <b>158</b> compared to <figref idrefs="DRAWINGS">FIG. 8</figref>.

Finally, energy storage can also occur in the power additive mode using the flywheel <b>164</b> and/or accumulator(s) <b>160</b>/<b>162</b>. When braking is desired, power from the wheels <b>159</b> is transferred to the displacement unit <b>158</b> through the shaft <b>186</b> and to the flywheel <b>164</b> and displacement unit <b>156</b> through the shaft <b>168</b>. When storing energy with the HP accumulator <b>160</b>, both displacement units <b>156</b> and <b>158</b> operate as pumps to deliver fluid to the accumulator <b>160</b>. The freewheel clutch <b>166</b> allows the flywheel <b>164</b> to accelerate and rotate at speeds faster than the engine <b>151</b>.

A “Standard Control” scheme of the valve block <b>163</b> is represented on the lefthand side of <figref idrefs="DRAWINGS">FIG. 15</figref>. The valve block <b>163</b> includes a check valve V<b>1</b> that prevents the HP accumulator <b>160</b> from discharging when in position <b>0</b>, and a second valve V<b>2</b> that prevents the HP accumulator <b>160</b> from being charged when in position <b>0</b>. A third valve V<b>3</b> prevents high pressure flow from entering the LP accumulator <b>162</b> when in position <b>0</b>, and a check valve V<b>4</b> prevents high pressure flow from entering the LP accumulator <b>162</b>. <figref idrefs="DRAWINGS">FIG. 15</figref> does not show the low pressure system of the hydrostatic transmission system <b>152</b> or high pressure relief valves that are preferably employed to direct flow to the LP accumulator <b>162</b> if the HP accumulator <b>160</b> is filled. Various options could be considered for handling the low pressure fluid of the hydrostatic transmission system <b>152</b>.

A “Secondary Control” scheme is also represented in <figref idrefs="DRAWINGS">FIG. 15</figref>. With this scheme, the displacements of both displacement units <b>156</b> and <b>158</b> are controlled simultaneously by separate control signals. With the Secondary Control scheme, the differential system pressure is not simply a reactionary function of the load torque on the wheels <b>159</b>, but is regulated to always operate at or above an allowable minimum pressure level, p_1HP, chosen so that the displacement unit <b>158</b> can produce a minimum allowable amount of torque at the wheel axle. The high pressure line is always fluid line A during Secondary Control. Measurement of vehicle speed and pressure in line A are used as feedback signals.

The full mechanical point, v_mech, marks the transition from Standard Control to Secondary Control. At vehicle speeds (v_veh) below v_mech, the input-coupled PSD transmissions <b>150</b>, <b>250</b>, <b>350</b>, and <b>450</b> are controlled with the Standard Control scheme. v_mechoccurs at approximately 33% of the maximum vehicle velocity, and is described in Carl et al. (noted previously, and whose contents are incorporated herein by reference) as the vehicle speed at which the displacement unit <b>158</b> experiences zero rotational velocity: <br /><i>v</i>_mech≡(<i>n</i>_A/(1<i>−i</i>_o)·<i>i</i>_axle)·<i>r</i>_tire<br /> where n_Ais the rotational speed of the shaft <b>168</b>, i_ois the standing gear ratio of the planetary gear train <b>154</b>, i_axleis the gear ratio of the wheel axle, and r_tireis the dynamic rolling radius of the wheels <b>159</b>. The vehicle velocity feedback signal, v_veh, and shaft speed, n_A, are all that is required to indicate that v_mechhas been reached and the transition from Standard Control to Secondary Control can occur. For any speed above v_mech, the transmissions <b>150</b>, <b>250</b>, <b>350</b>, and <b>450</b> can be controlled with either Standard Control or Secondary Control, though energy recovery occurs only when the Secondary Control scheme is active. When the feedback signal v_vehis greater than or equal to the calculated value v_mechas defined in the equation above, the control scheme can be switched from Standard to Secondary Control. If the control scheme switch occurs, valves V<b>1</b> through V<b>3</b> are all energized.

The Power Flow Modes of the input-coupled PSD transmissions <b>150</b>, <b>250</b>, <b>350</b>, and <b>450</b> are summarized in <figref idrefs="DRAWINGS">FIGS. 14A through 14G</figref>. It should be noted that the HP and LP accumulators <b>160</b> and <b>162</b> are coupled to the fluid lines A and B through the valve block <b>163</b>, and not directly to the fluid lines A and B as shown in the simplified schematics used in <figref idrefs="DRAWINGS">FIGS. 14A through 14G</figref>. Each Power Flow Mode has a unique set of control signals, one signal for each unit.

During Modes <b>0</b> through III, under Standard Control the displacements of the displacement units <b>156</b> and <b>158</b> are controlled sequentially to achieve a desired vehicle speed, v_veh,des. During Mode IV under Secondary Control, the vehicle is propelled with engine power, the displacement of the unit <b>156</b> is controlled to maintain p_1HPin the HP accumulator <b>160</b>, and the displacement of the unit <b>158</b> is simultaneously controlled to achieve the desired vehicle speed, v_veh,des. The torque output of the unit <b>158</b> can be adjusted by changing the displacement of the unit <b>158</b>, such that the resulting aiding torque on the wheel axle can be increased as necessary to attain the desired vehicle speed. During Mode V of the Secondary Control, the vehicle decelerates, the displacement of the unit <b>156</b> is controlled to regulate the speed of the shaft <b>168</b> to some desired speed, n_A,des, by exerting an appropriate amount of resistive torque on the shaft <b>168</b>, and the displacement of the unit <b>158</b> is simultaneously controlled to achieve a desired vehicle speed, v_veh,des, by exerting an appropriate amount of resistive torque on the axle shaft through the shaft <b>186</b>. As a result, the pressure level of the HP accumulator <b>160</b> increases as the vehicle's kinetic energy is stored in the form of pressurized fluid within the accumulator <b>160</b>.

During Mode VI of Secondary Control, the vehicle is propelled with hydraulic power. The displacement of the unit <b>156</b> is again controlled to regulate the speed of the shaft <b>168</b> to some desired speed, n_A,des, by exerting an appropriate amount of aiding torque on the shaft <b>168</b>. The engine speed is allowed to drop below n_A,des, while simultaneously the displacement of the unit <b>158</b> is again controlled to achieve a desired vehicle speed, v_veh,des, by exerting an appropriate amount of aiding torque on the axle shaft through the shaft <b>186</b>. As a result, the pressure level within the HP accumulator <b>160</b> decreases as the accumulator's potential energy is transferred to vehicle kinetic energy.

At the full mechanical point, v_mech, the input-coupled PSD transmissions <b>150</b>, <b>250</b>, <b>350</b>, and <b>450</b> switch naturally from power recirculation mode to power additive mode. Power from the wheels <b>159</b> travels through the planetary gear train <b>154</b> during braking maneuvers. During braking in power recirculation mode, the unit <b>158</b> operates as a motor and does not transfer energy to the HP accumulator <b>160</b>. Similarly, although the unit <b>156</b> is operating as a pump during this time, it does not transfer energy to the HP accumulator <b>156</b>. Consider that power enters the planetary gear train <b>154</b> through the shaft <b>178</b> during recirculation mode braking. If the engine <b>151</b> is incapable of storing energy due to the freewheel clutch <b>166</b>, all power transferred to the unit <b>156</b> from the shaft <b>168</b> must be transferred to the shaft <b>186</b> through the unit <b>158</b> in order to balance the power flow within the planetary gear train <b>154</b>, that is, power into the planetary gear train <b>154</b> must be equal to the power out of the planetary gear train <b>154</b>. Storing energy in the HP accumulator <b>160</b> requires that not all power from the shaft <b>168</b> is transferred back into the shaft <b>186</b>, creating a power unbalance within the planetary gear train <b>154</b>. Therefore energy storage during power recirculation mode braking is not possible. During power additive mode, the units <b>156</b> and <b>158</b> operate as pumps during braking maneuvers and are capable of transferring energy to the HP accumulator <b>160</b>. Power and therefore energy transferred into the planetary gear train <b>154</b> through shaft <b>178</b> is transferred to the HP accumulator <b>160</b> through the units <b>156</b> and <b>158</b> via the shafts <b>168</b> and <b>186</b>, respectively, such that the power balance within the planetary gear train <b>154</b> is satisfied. Since energy capture is only possible during power additive mode, it is advantageous to decrease the engine speed allowing for a lower value of n_A,desduring braking maneuvers to extend the energy capture region as much as possible, since lowering n_Alowers v_mech.

The potential for fuel savings through the use of the output-coupled and input-coupled PSD transmissions of <figref idrefs="DRAWINGS">FIGS. 4 and 9</figref> were modeled using a software library entitled Power Split Drive Design (PSDD), built in a Matlab/Simulink environment and reported in Mikeska et al., “Virtual Prototyping of Power Split Drives,” Proc. Bath Workshop on Power Transmission and Motion Control PTMC (2002), Bath, UK, p. 95-111, whose contents are incorporated herein by reference. The model simulated the use of a reference vehicle with a 225 kW engine and a maximum driving speed of 160 km/h under an urban dynamometer driving cycle. At the conclusion of the simulation, the output-coupled PSD transmission <b>110</b> of <figref idrefs="DRAWINGS">FIG. 4</figref> was indicated as being the most advantageous solution in terms of energy savings for the reference vehicle and the studied drive cycle. As compared to a conventional input-coupled PSD transmission (<figref idrefs="DRAWINGS">FIG. 8</figref>), the output-coupled and input-coupled PSD transmissions <b>110</b> and <b>150</b> of this invention were predicted to have energy efficiency savings of about 35.27% and about 16.19%. The output-coupled PSD transmission <b>110</b> further showed a distinct advantage over the input-coupled PSD transmission <b>150</b> in its ability to capture energy at all speed ranges during the simulated drive cycle. This result may be attributable to the direct connection between the displacement unit <b>118</b> and the wheel axle that exists in the output-coupled system, eliminating the need for power flow to exist in the planetary gear train <b>114</b> during energy capture maneuvers. The input-coupled system was limited to energy capture only at vehicle speeds greater than v_mech, roughly 33% of the maximum velocity.

While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the PSD transmissions could differ from those shown, and components capable of function similar to the components described could be use. Therefore, the scope of the invention is to be limited only by the following claims.